THE EPITHELIAL TISSUES. 



able for the propulsion of mucus or secretions, the free surface of the epi- 

 thelium in many localities, as in the trachea and bronchial tubes, the inferior 

 and middle nasal meatuses and the oviducts and uterus, is provided with 

 minute hair-like vibratile processes, or cilia. The exact relations of the cilia, 

 specializations of the substance of the cell, to the cytoplasm are still uncertain' 

 although it is probable that the hair-like processes attached to the free 

 surface are connected with delicate intracellular fibrillae within the superficial 

 and more highly specialized parts of the cells. In man and the higher 

 mammals ciliated epithelium is limited to the columnar variety. The 

 number of cilia attached to each cell varies, but there are usually between 

 one and two dozen such appendages. Their length, likewise, differs, those 

 lining the epididymis being about ten times longer than those attached to 

 the tracheal mucous membrane. Under favorable conditions, including a 

 sufficient supply of moisture, oxygen and heat, ciliary motion may continue 

 for many hours or even days after removal of the tissue. 



The cytoplasm of epithelial cells often is invaded by particles of foreign 

 substances; thus, granules of fatty and proteid matters are very common, 

 while the presence of granules of keratohyalin in certain cells of the epidermis 



Ciliated 

 border 



FIG. 22. 



Stratified ciliated columnar epithelium 

 from trachea. X 500. 



FIG. 23. Ciliated epithelial cells. A, 

 from intestine of a mollusk ; /?, from nasal 

 mucosa of frog. X 675. (Engflmann.) 



characterizes the stratum granulosum. When the contained particles are 

 colored, as when composed of melanin, the affected cells acquire a dark 

 brown tint and are then known as pigmented epithelium. Examples of such 

 cells are seen in the outer layer of the retina and in deep cells of the epider- 

 mis in certain races. 



On surfaces clothed with columnar epithelium, many cells are distin- 

 guished by unusually clear cytoplasm and exceptional form and size. These 

 are the goblet-cells, whose peculiar chalice form results from an accumulation 

 of mucoid secretion elaborated within the cytoplasm of the cells. When dis- 

 tention becomes too great, the cell ruptures in the direction of least resist- 

 ance and the secretion is poured out upon the surface of the mucous mem- 

 brane as the lubricating mucus. The goblet-cells, therefore, may be 

 regarded as unicellular glands and as representing the simplest phase in the 

 temporary specialization of glandular tissue. When the epithelial elements 

 become permanently modified to engage in the elaboration of secretions, 

 they are recognized as glandular epithelium. The cells lining the ducts and 

 the ultimate compartments of the glands are modified extensions of the epi- 

 thelium covering the adjoining mucous membrane. Their form and condi- 

 tion depend upon the degree of specialization, varying from columnar to 



